There has been known a technique in which when connecting a plurality of battery packs in parallel, in a case where there is a voltage difference between each of cells, in order to prevent an excessive inrush current from flowing from a higher voltage cell to a lower voltage cell, the cells are connected in parallel after the lower voltage cell is charged so that potential difference becomes a predetermined level or less (see e.g. Patent Document 1).
Here, when the excessive inrush current flows in the cell upon parallel-connecting the plurality of cells, there is a possibility that life of the battery (the cell) will be shortened and/or heat generation of the battery (the cell) will occur. Thus, while it is required that the excessive inrush current be surely prevented from flowing in the cell, a method of parallel-connecting the plurality of cells in a short time is desired.
However, in the case of a method disclosed in the Patent Document 1, since the lower voltage cell is charged and its voltage is made equal to voltage of the higher voltage cell, a process (an operation) to connect the cell to a charger and also a process (an operation) to charge the cell are needed, then there arises a problem of taking time required to complete the parallel-connection of the cells.